The aim of this work was to confirm previous findings suggesting that the eyes are required for night-time melatonin production in Nile tilapia and further characterise this divergent circadian organisation. To do so, melatonin levels were firstly measured in eyecups and plasma to determine circadian patterns of melatonin production. Secondly, the effect of partial ophthalmectomy on the suppression of melatonin production was determined in vivo as well as ex vivo pineal light/dark sensitivity. Finally, to investigate whether such findings could be related to post-surgery stress, melatonin analyses were performed in the subsequent 24 h and 7 days post- ophthalmectomy with cortisol levels assessed as an indicator of stress. Our results showed an inverse pattern of melatonin production in the eye cups of tilapia compared to blood circulating levels, suggesting different roles played by melatonin in these two tissues. Results then demonstrated that total or partial ophthalmectomy resulted in the suppression of night-time melatonin production. Furthermore, although pineals in culture were shown to be photosensitive, night-time melatonin levels were much lower than seen in other species. Finally, when performing sampling immediately or one week post-surgery, no difference in the melatonin profiles were observed. It is therefore unlikely that post-surgery stress would explain such suppression in melatonin production although all fish displayed high cortisol levels most probably due to social and handling stress. Taken together, these results provide further evidence of a new type of circadian organisation in a teleost species where the eyes are required to sustain night time melatonin levels.

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